1. Field of the Invention
The present invention relates to a gas analyzing apparatus, including a contained oxygen analyzing apparatus and a contained oxygen analyzing method which can measure a trace amount of contained oxygen by removing an oxide film on a sample surface to enable a measurement of a small amount of oxygen in a sample to be measured, for example, a metal sample (particularly, steel).
2. Description of Related Art
As a method of quantitatively analyzing an oxygen contained in steel, there has been generally used a method of combining a fusion extraction method in an inert gas, an infrared absorbing method and a thermal conductivity method. The fusion extraction method, the infrared absorbing method and the thermal conductivity method comprise the steps of arranging a graphite crucible in which a sample to be measured is inserted, within a heating furnace, heating and fusing the steel corresponding to the sample to be measured while supplying an inert gas, and analyzing carbon monoxide or carbon dioxide generated at this time, for example, by an infrared gas analyzer.
In order to accurately detect the oxygen contained at only a trace quantity in the sample such as steel, it is necessary to previously remove any oil content, dirt or the like attached to a surface of the sample (hereinafter, refer to as an attachment), and any oxide film. Further, in order to remove any attachment attached to the surface, a preliminary treatment was applied to the sample by heating the sample at 400° C. to 600° C. for about ten minutes.
FIGS. 13a and 13b show one example of a conventionally executed contained oxygen analyzing method. The method of removing the oxide film shown corresponds to a method described in Japanese Unexamined Patent Publication No. 6-148170. FIG. 13A shows the temperature change within a carbon furnace (a graphite crucible) in correspondence to a procedure of contained oxygen analysis, and FIG. 13B shows the change in amount of signals detected by the infrared gas analyzer.
First, the carbon furnace is preliminarily heated at a high temperature, for example, 3000° C. or the like, between time points Tp11 and Tp12 in FIG. 13A. Next, the surface oxide film is reduced by inputting the sample to be measured into the graphite crucible in which the preliminary heating is finished, at a time point Tp13, and heating the sample to be measured to a temperature equal to or less than a melting point (for example, in a range between 900° C. and 1400° C.), between time points Tp14 and Tp15. Then, an amount of the contained oxygen in the sample to be measured is analyzed by increasing a temperature of the carbon furnace to be equal to or more than 1400° C. (in particular, 2400° C.).
FIGS. 14A and 14B show another example of a contained oxygen analyzing method, in which FIG. 14A shows a temperature change within the carbon furnace in correspondence to a procedure of the contained oxygen analysis, and FIG. 14B shows a change in mount of signals detected by the infrared gas analyzer.
In accordance with the method shown, first, the graphite crucible is preliminarily heated between time points Tp21 and Tp22, and thereafter, the sample to be measured is input into the graphite crucible at a time point Tp23. Next, the surface oxygen is removed by heating the sample to be measured, for example, to 1050° C. in the graphite inert gas between time point Tp24 and Tp25. Then, the sample to be measured is cooled in the inert gas to be approximately room temperature, and at a time point Tp26 the sample to be measured is taken out to the ambient air so as to be oxidized.
Next, the sample to be measured is input into the graphite crucible at a time point Tp29, after a preliminary heating is again applied to the graphite crucible between time points Tp27 and Tp28. The surface oxygen is again removed by heating the sample to be measured, for example, to 1050° C. in the graphite inert gas between time points Tp30 and Tp31, and the amount of oxidation of the oxide film is measured from a signal amount Sp21 at this time. Then, the sample is cooled to approximately room temperature in the inert gas, at a time point Tp32 and the sample is taken out to ambient air so as to be again oxidized.
Further, after the preliminary heating is applied for a third time between time points Tp33 and Tp34, a metal solvent is input into the graphite crucible at a time point Tp35, and a metal bath of the metal flux is prepared in the graphite crucible by heating an inner side of the graphite crucible, for example, to 2400° C. Then, a signal amount Sp22 of a gas generated by inputting the sample into the graphite crucible is measured at a time point Tp36. Accordingly, a contained oxygen amount (a bulk oxygen) of the sample is determined by subtracting the oxygen amount of the oxide film calculated from the signal amount mentioned above from a whole oxygen amount to be calculated (Sp22−Sp21).
However, in accordance with the conventional oxygen analyzing method shown in FIGS. 13A and 13B, since the signal Sp11 caused by the carbon monoxide gas generated from the graphite crucible is increased at a time of increasing the temperature of the graphite crucible from a preliminary heating temperature between 900 and 1400° C. to 1400° C. or more (in particular, 2400° C.), an influence of fluctuation in the signal Sp11 is added to the measured signal amount Sp13, so that there is a problem that a magnitude of the signal Sp12 caused by the oxygen contained in the sample to be measured cannot be accurately determined.
Especially, since the increase of the signal Sp11 caused by the carbon monoxide gas generated from the graphite crucible creates an influence because the oxygen contained in the sample to be measured is only an extremely trace quantity, and it is impossible to measure below 0.5 ppm at the maximum, for example, with respect to an indicated value of 2.9 ppm. That is, in accordance with the conventional oxygen analyzing method shown in FIG. 13, it is impossible to analyze an extremely small quantity of contained oxygen.
Further, in accordance with the conventional oxygen analyzing method shown in FIG. 14, it is necessary to measure the surface oxidation oxygen at two times, so that it is unavoidable to increase the time of measurement. In addition, the subtraction (Sp22−Sp21) is executed on the assumption that the amount of the surface oxidation oxygen at the first time is equal to the amount of the surface oxidation oxygen at the second time; however, it is unavoidable that both amounts fluctuate due to the time in contact with the ambient air or the other conditions. That is, in accordance with this example, it is impossible to restrict a dispersion of the surface oxidation to be equal to or less than 0.5 ppm at the maximum with respect to the value of analysis in the case of measuring a trace quantity of contained oxygen of about 2.9 μg/g.